Recent advances in size reduction or higher performances of electronic devices have been necessitating efficient heat radiation from semiconductors-mounted circuit substrates or various chips- or devices-mounted electronic parts, as an important problem. Accordingly, highly heat-conductive materials have been now used in adhesion of various electronic parts or coatings onto circuit substrates.
Such materials include sheet-shaped moldings and paste-state adhesives. The sheet-shaped moldings involve such problems that adhesion must be conducted through pressure fitting parts of such as clips, bolts, etc. or upon tight contact thereof on desired positions through a sticking agent, etc. The paste-state adhesive involves such problems that the curing requires a relatively long time or elevated temperatures, or furthermore that the viscosity is so high due to a higher content of a heat-conductive filler, thereby the workability will be deteriorated.
To improve the production efficiency of semiconductor devices on the basis of flip chip bonding, epoxy resin compositions have been so far proposed, that can form cured resins having short curing time on pressure welding process and having voidless and distinguished adhesiveness. Concerning epoxy resin composition in a liquid state at room temperature, comprising an epoxy resin and a curing agent, it is known that the viscosity of the epoxy resin composition can be lowered by using an addition reaction product of 2-methylimidazole and 3,4-epoxycyclohexanecarboxylic acid-3′,4′-epoxycyclohexylmethyl ester, i.e. an alicyclic epoxy compound, as the curing agent.    [Patent Document 1] JP-A-2006-8854
Epoxy resin for use in the afore-mentioned epoxy resin composition includes, for example, bisphenol F type epoxy resin, bisphenol A type epoxy resin, naphthalene ring-structured epoxy resin, hydrogenated epoxy resins thereof, alicyclic epoxy resin, etc., and it is further known that bis(3,6-diglycidyloxynaphthyl)methane, i.e. a naphthalene ring-structured tetrafunctional epoxy resin, can be used together with these epoxy resins in a proportion of 5-40% by equivalent weight in terms of epoxide groups on the basis of total epoxy resins, and also known that electroconductive particles, e.g. polystyrene-based polymer particles with gold plated nucleus surfaces, can be further contained thereto. Patent Document 1 mentions a drastic increase in the curing speed, but fails to teach both of lowering of curing temperature and on increase in storage stability.
In the meantime, highly heat-conductive fillers having a heat conductivity such as 0.5 W/m·K, or more, for example, Al2O3, MgO, BN, AlN, Al(OH)3, Mg(OH)2, SiC, etc. have been so far used as a highly heat-radiatable materials to meet the recent requirements for size reduction and higher performance of electronic devices. In other words, epoxy resins containing a large amount of these highly heat-conductive fillers have been used as highly heat-radiatable materials.
To allow the epoxy resins to contain a large amount of the highly heat-conductive fillers, usually a large amount of a dispersing agent must be used. These dispersing agents are, when used together with a latent curing agent for the epoxy resin, involves such problems as deterioration of curability of epoxy resin composition or extreme deterioration of storage stability.
A resin composition for sealing, which comprises (A) epoxy resin, (B) a curing agent for epoxy resin which includes phenol resin, acid anhydride or amine-based compound, (C) an elastomer, and (D) surfactant, and for use in combination with flip chip bonding, has been so far proposed with such disclosure that the composition can contain a filler such as silica, alumina, silicon nitride, mica, white carbon, etc., or a curing promoter such as imidazole-based compounds, etc. to adjust a linear expansion coefficient of the resin composition, and with such further recommendation that the surfactant may be any of an anionic surfactant, a cationic surfactant, a non-ionic surfactant, and an amphoteric surfactant, and preferably polyoxyalkylene-modified polydimethylsiloxane as a non-ionic surfactant, because of the presence of no adverse effect on the electric characteristics of sealing agents or semi-conductors, even if the surfactant is contained in a necessary proportion (i.e. 0.01-1 parts by weight, preferably 0.05-0.5 parts by weight, on the basis of 100 parts by weight of sum total of the epoxy resin and the curing agent).    [Patent Document 2] JP-A-2004-75835
However, it is disclosed that its effect is prevention of crack generation in underfillings or peeling from substrates, even if used in combination of a solder, which requires reflow at a higher temperature than the conventional, such as a lead-free solder, and that such effect can be obtained when both of the elastomer and the surfactant are contained.
Patent Document 2 makes no mention of storage stability or heat curability. In other words, it is desirable that the epoxy resin composition for use in the relevant technical field has a good storage stability and can be cured for a relatively short time or at a lower temperature, and thus it is important that both of a good storage stability and a low-temperature, short-time curability can be obtained at the same time.